Signal Transduction Scf Synergy

Multiple pathways and effectors have been implicated in intracellular signaling downstream from c-kit activation and phosphorylation (168). These pathways/effectors include phosphatidylinositol 3 kinase (Pl3K)/Akt; Janus Kinase/signal transducers and activators of transcription (JAK/STAT); Ras/Raf/mitogen-activated protein kinase (MAPK); Src-family kinases; protein kinase C (PKC); and phospholipase c-y (PLC-y)/inositol phosphates/diacylglycerol. Considerable progress has been made in identifying Tyr and Ser/Thr residues in the cytoplasmic portion of c-kit whose phos-phorylation/dephosphorylation dictate initial intracellular molecular interactions with c-kit; integration and crosstalk between pathways, including pathways initiated by other cytokines and regulators; functional consequences of signaling through different pathways; and differences in signaling pathways and signaling consequences as a function of cell lineage and stage of maturation.

Additive and complementary intracellular signal transduction that promotes cell cycling and proliferation and inhibits apoptosis is probably the major mechanism by which the synergies of SCF with other cytokines and effectors come about (168). Various studies illustrate signaling routes potentially facilitated by SCF:

1. In the MO7e human megakaryocyte leukemia cell line, SCF synergy with GM-CSF may represent enhancement of Ras-Raf-MAPK signaling that promotes cell cycling and proliferation by induction of early response genes (c-Fos, JunB, c-Myc) and induction of the cyclin-dependent kinase (Cdk) inhibitor p21cip-1 (such that Cdk2 activity and phosphorylation of retinoblastoma protein [Rb] is increased) (168,169). Similarly, in purified human erythroid progenitor cells, markedly synergistic activation of MAPK by EPO and SCF has been shown (170).

2. MO7e-G cells (MO7e cells transfected with the G-CSF receptor) proliferate addi-tively/synergistically in response to SCF and G-CSF (171). The combination of SCF and G-CSF, compared with each alone, leads to enhanced proliferation associated with c-fos induction, downregulation of p27kip-1 (Cdk inhibitor), hyperphosphorylation of Rb, phosphorylation-based hyperactivation of STAT3, and shortening of G0/G1 time; STAT3 may serve as a point of integration for signaling through the Ras-Raf-MAPK and PI3K/Akt pathways in these cells (171).

3. In the HB60-5 murine erythroblastic cell line, which proliferates in response to SCF plus EPO in combination and differentiates in response to EPO alone, SCF appears to keep the cells in a proliferating, cycling, undifferentiated state by downregulation of p27kip-1

and p15 (Cdk inhibitors), upregulation of Cdc25A (phosphatase that activates Cdks), and associated upregulation of c-Myc (172)

4. In purified human erythroid progenitor cells, SCF can inhibit apoptosis by means of PI3K activation and Akt phosphorylation (173); and by means of uncoupling Fas ligation from caspase activation (174,175) in a way that is dependent on Src-family kinase(s) (174) and may occur through upregulation of an inhibitor of FLICE (caspase-activating enzyme) (175).

5. In some cell types, SCF appears to inhibit apoptosis by means of effects on Bcl family members, e.g., phosphorylation (and inactivation) of the proapoptotic protein Bad (176) and upregulation of the antiapoptotic protein Bcl-2 (111).

SCF synergy also may occur by crosstalk with other cytokine receptors. For example, SCF by itself stimulates Ser/Thr phosphorylation of the IL-3 receptor P-subunit in a murine bone marrow-derived cell line (168). In the murine erythroid progenitor cell line HCD57, SCF by itself stimulates not only Tyr phosphorylation of the EPOR (177,178), but also physical association between c-kit and the EPO receptor (177,179); however, the functional consequences of these receptor interactions are not completely clear (178,180,181). SCF-stimulated phosphorylation of the EPO receptor has been seen with purified human erythroid progenitor cells (170).

SCF can influence the adhesion properties of various cell types. In hematopoietic cells, SCF can increase the avidity of the integrins very late antigen (VLA)-4 and VLA-5 for partners such as vascular cell adhesion molecule-1 (VCAM-1) and fibronectin (5). These adhesion molecule interactions can in turn mediate intracellular proliferative signals.

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